The long range goal of our laboratory is to understand the structural basis for actomyosin contractility and motility and its regulation at the molecular level. The ability of muscles to contract,k cells to under go division and shape changes are fundamental biological processes based in part or whole on the actomyosin interaction which is necessary for any organism to survive. Although much information is known about these processes and their regulatory control, little is known of the structural changes responsible for force generation and movement and its regulation in terms of the molecular structure of myosin or actomyosin. The present proposal seeks to characterize the location and extent of the structural changes which occur in the S1 region of the myosin crossbridge when it interacts with MgATP and actin. This will be done by examining the proximal relationship in S1 in different conformational states associated with the contractile cycle by employing heterofunctional, photoactivatable reagents to modify the protein. This will allow for the characterization of regions which remain static or which move during the contractile cycle. In addition the substructure of S1 will be studied by examining how the protein unfolds and refolds in the presence of nucleotide or actin. This will be done by employing specific modification of certain side-chains of the protein and by the use of antibodies to follow the unfolding and refolding process. This project should provided detailed information about the organization of the structure of S1 and the changes which it undergoes to produce contraction in muscle and actomyosin based motility in non- muscle cells.